insanity question

I've started my third week yesterday [this is my second round doing insanity...I waited 6-ish weeks before doing insanity again]

Anyway, for some reason I've gained a little bit of weight [a little bit less than a pound], but whenever I weigh in the day after the rest day, I'm down a pound or more. Then the day after I start exercising again, I'm back up a pound. Just saying, but I eat the same amount of food every single day